{"id":247,"date":"2018-06-05T14:07:29","date_gmt":"2018-06-05T18:07:29","guid":{"rendered":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/chapter\/7-3-classification-of-igneous-rocks-2\/"},"modified":"2023-06-20T13:28:11","modified_gmt":"2023-06-20T17:28:11","slug":"classification-of-igneous-rocks","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/chapter\/classification-of-igneous-rocks\/","title":{"raw":"7.3 Classification of Igneous Rocks","rendered":"7.3 Classification of Igneous Rocks"},"content":{"raw":"Igneous rocks are classified (named) based on two sets of characteristics:\r\n
    \r\n \t
  1. The minerals they contain<\/li>\r\n \t
  2. Their grain size and texture<\/li>\r\n<\/ol>\r\n

    Classification By Mineral Abundance<\/h1>\r\nIgneous rocks can be divided into four categories based on their chemical composition: felsic, intermediate, mafic, and ultramafic. The diagram of Bowen's reaction series (Figure 7.7) shows that differences in chemical composition correspond to differences in the types of minerals within an igneous rock.\u00a0 Igneous rock names are based in part on the minerals that igneous rocks contain, and on how abundant they are in a rock.\r\n\r\nFigure 7.13 is a diagram with the minerals from Bowen's reaction series, and it's used to decide which name to give an igneous rock. First, notice that the diagram has rows with different kinds of information, but is also organized in columns according to the four compositional categories. The top box shows the range of mineral proportions for each compositional category. An igneous rock can be represented as a vertical line drawn through the top box of the diagram, and the vertical scale\u2014with the distance between each tick mark representing 10% of the minerals within a rock by volume\u2014is used to break down the proportion of each mineral it contains.\r\n\r\nConsider the arrows in the mafic field of the diagram. They represent a rock containing 60% pyroxene and 40% pyroxene. An igneous rock at the boundary between the mafic and ultramafic fields (marked with a vertical dashed line) would have approximately 20% olivine, 60% pyroxene, and 20% Ca-rich plagioclase feldspar by volume.\r\n\r\n[caption id=\"attachment_241\" align=\"aligncenter\" width=\"1024\"]\"\"<\/a> Figure 7.13<\/strong> Classification diagram for igneous rocks. Igneous rocks are classified according to the relative abundances of minerals they contain. A given rock is represented by a vertical line in the diagram. In the mafic field, the arrows represent a rock containing 60% pyroxene and 40% olivine. The name an igneous rock gets depends not only on composition, but on whether it is intrusive or extrusive. Source: Karla Panchuk (2018), CC BY-NC-SA 4.0. Modified after Steven Earle (2015), CC BY 4.0. and others. Click for attributions and more information.[\/caption]\r\n

    Classification By Grain Size<\/h1>\r\nThe lower two boxes of the diagram contain igneous rock names, and you'll notice that there are two igneous rock names for each compositional category. So which name do you use?\r\n\r\nThe name to choose depends on whether the igneous rock cooled within Earth (whether it's an intrusive<\/strong> or plutonic<\/strong> igneous rock), or whether it cooled on the Earth's surface after erupting from a volcano (making it an extrusive<\/strong> or volcanic<\/strong> igneous rock).\r\n\r\nWhat this means is that two igneous rocks comprised of exactly the same minerals, and in the exactly the same proportions, can have different names. A felsic intrusive<\/em> rock is called granite<\/strong>, whereas a felsic extrusive<\/em> rock is called rhyolite<\/strong>. Granite and rhyolite have the same mineral composition, but their grain size gives each a distinct appearance. A rock of intermediate composition is diorite<\/strong> if it's course-grained, and andesite<\/strong> if it's fine-grained.\u00a0 A mafic rock is gabbro<\/strong> if it's course-grained, and basalt<\/strong> if fine-grained. The course-grained version of an ultramafic rock is peridotite<\/strong>, and the fine-grained version is komatiite<\/strong>. It makes sense to use different names because rocks of different grain sizes form in different ways and in different geological settings.\r\n
    \r\n\r\n<\/a>Try It Out!<\/strong>\r\n
    An igneous rock is shown below, along with the abundances of minerals within it. Answer the questions to figure out what kind of rock it is.\"\"\r\n
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    1. This rock has quartz in it. Based on Bowen's reaction series only, which two compositional categories can you rule out for sure?\r\n
        \r\n \t
      1. Felsic<\/li>\r\n \t
      2. Intermediate<\/li>\r\n \t
      3. Mafic<\/li>\r\n \t
      4. Ultramafic<\/li>\r\n<\/ol>\r\n<\/li>\r\n \t
      5. Next, look at the mineral abundances more carefully. Is this rock felsic or intermediate? Hint: When you look at the compositional diagram, what do you notice about the type and abundance of feldspar in these categories?<\/li>\r\n \t
      6. Is this rock intrusive or extrusive? Hint: can you see individual crystals?<\/li>\r\n \t
      7. Now put it all together. Which of these is an intermediate intrusive igneous rock?\r\n
          \r\n \t
        1. Andesite<\/li>\r\n \t
        2. Rhyolite<\/li>\r\n \t
        3. Diorite<\/li>\r\n \t
        4. Granite<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\nTo check your answers, navigate to the below link to view the interactive version of this activity.\r\n\r\n<\/div>\r\n[h5p id=\"74\"]\r\n\r\n<\/div>\r\n

          What Determines Grain Size?<\/h2>\r\nThe key difference between intrusive and extrusive igneous rocks\u2014the size of crystals making them up\u2014is related to how rapidly melted rock cools. The longer melted rock has to cool, the larger the crystals within it can become.\u00a0 Magma cools much slower within Earth than on Earth's surface because magma within Earth is insulated by surrounding rock.\u00a0 Notice that in Figure 7.13, the intrusive rocks have crystals large enough that you can see individual crystals\u2014either by identifying their boundaries, or seeing light reflecting from a crystal face.\u00a0 A rock with individual crystals that are visible to the unaided eye has a phaneritic<\/strong> or coarse-grained<\/strong> texture. The extrusive rocks have much smaller crystals.\u00a0 The crystals are so small that the bulk of individual crystals cannot be distinguished, and the rock may look like a dull mass. A rock with crystals that are too small to see with the unaided eye has an aphanitic<\/strong> or fine-grained<\/strong> texture.\u00a0 Table 7.1 summarizes the key differences between intrusive and extrusive igneous rocks.\r\n\r\n\r\n\r\n\r\n\r\n\r\n
          Table 7.1 Comparison of Intrusive and Extrusive Igneous Rocks<\/caption>\r\n
          <\/td>\r\nMagma<\/span> cools within Earth<\/th>\r\nLava<\/span> cools on Earth's surface<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
          Terminology<\/th>\r\nIntrusive\/ plutonic<\/td>\r\nExtrusive\/ volcanic<\/td>\r\n<\/tr>\r\n
          Cooling rate<\/th>\r\nSlow: surrounding rocks insulate the magma chamber.<\/td>\r\nRapid: heat is exchanged with the atmosphere.<\/td>\r\n<\/tr>\r\n
          Texture<\/th>\r\nPhaneritic<\/strong> (coarse-grained): individual crystals are large enough to see without magnification.<\/td>\r\nAphanitic<\/strong> (fine-grained): crystals are too small to see without magnification.<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n
          \r\n\r\n<\/a>Practice with Igneous Rock Names<\/strong>\r\n\r\n[h5p id=\"7\"]\r\n\r\n<\/div>\r\n

          Does This Mean an Igneous Rock Can Only Have One Grain Size?<\/h2>\r\nNo. Something interesting happens when there is a change in the rate at which melted rock is cooling.\u00a0 If magma is cooling in a magma chamber, some minerals will begin to crystallize before others do.\u00a0 If cooling is slow enough, those crystals can become quite large.\r\n\r\nNow imagine the magma is suddenly heaved out of the magma chamber and erupted from a volcano.\u00a0 The larger crystals will flow out with the lava. The lava will then cool rapidly, and the larger crystals will be surrounded by much smaller ones.\u00a0 An igneous rock with crystals of distinctly different size (Figure 7.14) is said to have a porphyritic<\/strong> texture, or might be referred to as a porphyry<\/strong>.\u00a0 The larger crystals are called phenocrysts<\/strong>, and the smaller ones are referred to as the groundmass<\/strong>.\r\n\r\n[caption id=\"attachment_242\" align=\"aligncenter\" width=\"531\"]\"\" Figure 7.14<\/strong> Porphyritic rhyolite with quartz and potassium feldspar phenocrysts within a dark groundmass. Porphyritic texture (when different crystal sizes are present) is an indication that melted rock did not cool at a constant rate. Source: Karla Panchuk (2018), CC BY-NC-SA 4.0. Photo by R. Weller\/Cochise College (2011). Image source.<\/a>[\/caption]\r\n\r\n
          \r\n\r\n<\/a>Which Phenocrysts Will Form?<\/strong>\r\n
          \r\n\r\nAs a magma cools below 1300\u00b0C, minerals start to crystallize within it. If the magma is then erupted, the rest of the liquid will cool quickly to form a porphyritic texture: the rock will have some relatively large crystals (phenocrysts) of the minerals that crystallized early, and the rest will be very fine-grained or even glassy.Refer to the diagram shown here, and use the checklists that follows to predict which phenocrysts might be present if the magma cooled as far as Line a before erupting, and which would be present if the magma cooled to Line b before escaping.\r\n\r\n\"Bowen's\r\n
            \r\n \t
          1. Checklist 1. Which phenocrysts might be present if magma cools as far as Line a? (Select all that apply.)\r\n